Working with quickly, are there any gtk.Builder code sketching tools that integrate?
That might be nice. It shouldn't be too difficult to implement either, since the handlers are defined in the ui-file. I don't know of any tools that does it now. I don't think it could be done in the GUI designer, since it's language independent. But adding a 'sketch' command to Quickly should be possible. If you add a wishlist bug to Quickly, perhaps someone can get around to it? I might want to have that myself.
https://launchpad.net/quickly